Demonstration of an absolute Rayleigh scattering spectral calibration on the W7-X Thomson scattering system

A novel in-situ calibration method for Thomson scattering (TS) diagnostics has been demonstrated on the Wendelstein 7-X stellarator (W7-X). Utilizing a laser-pumped optical parametric oscillator (OPO), Rayleigh scattering measurements were performed in-situ in argon, nitrogen, and air at pressures from 0 to 1024 mbar and wavelengths between 730 and 1070 nm. As the OPO follows the same beam path as the TS lasers, the scattering angle and solid angle of collection can be made identical to the TS geometry; therefore, the calibration covers all optical components within the TS system, including the first vacuum window, and can, in principle, be used to directly calibrate each filter channel for absolute plasma density. Despite high background stray light levels, the Rayleigh scattered signal was successfully isolated from the stray light, and filter curves for multiple polychromators were resolved. The complete calibration system, planned for future W7-X operational campaigns, will aim to further reduce stray light, thereby decreasing the required gas pressure and allowing in-situ calibration measurements on service days. This is planned in part by better matching the OPO beam alignment, size, and divergence to the installed Nd:YAG TS lasers. In addition, the OPO bandwidth will be improved by using an injection seeder to increase filter edge resolution.